Expansion processes have been widely used for hydrocarbon liquids recovery in the gas processing industry and are generally preferred for high ethane and propane recovery. External refrigeration is normally required in such processes where the feed gas contains significant quantities of propane and heavier components. For example, in a typical turbo-expander plant, the feed gas is cooled and partially condensed by heat exchange with process streams and/or external propane refrigeration. The condensed liquid containing the less volatile components is then separated and fed to a fractionation column, which is operated at medium or low pressure. The remaining vapor portion is letdown in pressure in a turbo-expander to a still lower pressure, resulting in further cooling and liquid formation. With the expander discharge pressure typically at demethanizer pressure, the two-phase stream is fed to the top of the demethanizer with the cold liquids acting as the top reflux to absorb the heavier hydrocarbons. The remaining vapor combines with the column overhead as a residue gas, which is then heated and recompressed to pipeline pressure.
However, in many expander plant configurations, the residue vapor from the fractionation column still contains a significant amount of ethane or propane plus hydrocarbons that could be recovered if chilled to a lower temperature, or subjected to a rectification stage. While lower temperature can be achieved with a higher expansion ratio across the turbo-expander, various disadvantages arise. Among other things, higher expansion typically results in lower column pressure and higher residue gas compression horsepower requirements, making high recovery uneconomical. Therefore, many NGL recovery configurations employ an additional rectification column, and use of a colder and leaner reflux stream to the fractionation column overhead vapor (see below). Furthermore, most known NGL recovery configurations are optimized for a single mode of operation (i.e., ethane recovery or propane recovery). Thus, when such NGL plants are required to switch recovery mode (e.g., from ethane to propane recovery), the efficiency and recovery levels tend to significantly drop. Still further, substantial reconfiguration and changes in operation conditions are necessary in most plants to achieve acceptable results. For example, most of the known plant configurations recover more than 98% of C3 and heavier hydrocarbons during the ethane recovery, but often fail to maintain the same high propane recovery during ethane rejection. In ethane rejection operation, the propane recovery levels from such processes often drop to about 90%, thereby incurring significant loss in product revenue.
Present NGL recovery systems can be classified into single-column configurations or two-column configurations, and some operating differences are summarized below. A typical single-column configuration for ethane recovery (which is also suitable for ethane rejection) is described in U.S. Pat. No. 4,854,955. Such configuration may be employed for moderate levels of ethane recovery due to the relatively low operating temperature and pressure of the fractionation column. In such plants, the column overhead vapor is cooled and condensed by an overhead exchanger using refrigeration generated from the feed gas chiller. This additional cooling step condenses the propane and heavier components from the column overhead gas, which is recovered in a downstream separator and returned to the column as reflux. For ethane rejection, this column operates as a deethanizer, and the pressure is typically lowered to about 350 psig to generate sufficient refrigeration from turbo-expansion and for the ethane/propane separation. However, the lower column pressure generally results in an increased residue gas compression horsepower demand. Other NGL recovery configurations that employ a single column for both ethane recovery and ethane rejection are described in U.S. Pat. No. 6,453,698. Here, an intermediate stream is withdrawn from the column to produce a lean vapor that is further cooled and condensed to generate a lean reflux to the column. While the heat integration, reflux configuration, and process complexity vary among many of these designs, all or almost all suffer from high energy consumption (e.g., due to the lower column pressure needed for cooling and fractionation).
Alternatively, a typical two-column NGL plant employs a reflux absorber and a second column that is operated as a demethanizer or deethanizer, which generally allows more flexibility in operating the absorber and the second column at different pressures. However, conventional two-column plants are generally only economic for either ethane recovery or propane recovery, but not both, and switching recovery modes will often incur significant propane losses (e.g., provide less than 98% plus C3 recovery).
For example, in U.S. Pat. Nos. 5,953,935 and 5,771,712, the overhead vapor or liquid from the second distillation column is recycled to the absorber as a lean reflux. While such plants provide relatively high ethane and propane recoveries, ethane rejection with high-yield propane recovery is often problematic under most operating conditions. Alternatively, as shown in U.S. Pat. No. 6,363,744, a portion of the residue gas stream from the residue gas compressor discharge is recycled as a lean reflux in the demethanizer. However, using residue gas to generate a cold reflux for the demethanizer consumes a large amount of horsepower, and the cost of residue gas compression is prohibitively high and usually not economical. Moreover, almost all of the above configurations require cryogenic operating temperatures for both the absorber and the distillation columns for ethane recovery operations, thereby increasing the capital cost of installation.
Thus, numerous attempts have been made to improve the efficiency and economy of processes for separating and recovering ethane and heavier natural gas liquids from natural gas. However, all or almost all of them fail to achieve economic operation when ethane rejection is required. Moreover, currently known configurations fail to provide flexibility in operation where recovery of ethane is only temporarily desired. Therefore, there is still a need to provide improved methods and configurations for flexible natural gas liquids recovery.